Two different methods of increasing the power of gas turbines based on cooling the air mass flow are known from the state of the art. Cooling of the air mass flow received by the gas turbine results in an increased power output by the machine. The reasons include first the increased mass flow of air due to the cooling and the reduction in power consumption by a compressor at low entrance temperatures. Fundamentally two types of cooling are known: cooling the inlet air flowing into the compressor and intermediate cooling between the two separate compressor stages.
For inlet cooling, different techniques are used, including methods that work with water spraying. When a small amount of water is sprayed (typically less than 1% of the air mass flow) and completely evaporates, cooling is achieved due to the uptake of heat of evaporation. On the other hand, cooling due to uptake of sensible heat in direct contact of air and water is possible. This is proposed in the unexamined German Patent DE-A1-199 13 681. To achieve a significant cooling, relatively large quantities of water are required here and a large contact area between the liquid phase and the air to be cooled is required. This can thus be achieved by atomization of the water into fine droplets. For example, DE-A1-199 13 681 proposes using a trickle cooler for this purpose. It should also be pointed out that an additional cooling may also occur with this type of cooling due to partial evaporation of the water. Whether evaporation occurs depends on the final temperature reached. If the final temperature is low enough, no water enters the vapor phase. Then it is even possible to condense water out of the intake air.
Use of water for inlet cooling limits the applicability of the method to sufficiently warm ambient temperatures at which there is no risk of freezing (approx. >10° C.). In corresponding heat exchangers with indirect cooling (water and air do not come in direct contact), there is fundamentally the possibility of counteracting the formation of ice by adding antifreeze. This possibility does not exist with direct spraying of water into the stream of air.
Intermediate cooling of the air during compression as is disclosed in DE-A1-42 37 665 reduces the power consumption of a turbo compressor as a result of reducing the compression work. In contrast with inlet cooling, however, no increase in the air mass flow is achieved in this way. In most cases, traditional heat exchangers are used for the intermediate cooling. In recent times however there have been increased efforts to achieve the desired cooling by spraying water. This is known for example from EP-A1-0 770 771. With this method, in the past the concept of cooling by vaporization has been pursued exclusively, whereby finely atomized demineralized water is sprayed into the air stream. This takes place either in the interior of the compressor between the individual compressor stages (so-called “spray intercooling”) or upstream from the compressor in the air intake. The amount of water added is normally low in these applications, namely between 0.5% and 1.5% of the mass flow of air. Cooling of the air here takes places almost exclusively through uptake of heat of evaporation by the water, but the exchange of sensible heat between gas and liquid water plays practically no role at all.
A considerable disadvantage of intermediate cooling by evaporation is the water loss. The liquid water sprayed in cannot in most cases be recovered economically from the exhaust and is discharged into the atmosphere. This can be a considerable problem, especially in regions of water shortage. With this type of intermediate cooling there are also technical problems due to the presence of liquid water in the compressor blades. This can lead to erosion and corrosion and also alters the aerodynamic properties of the blades.